In a commonly applied method of wellbore drilling, referred to as rotary drilling, a drill string is rotated by a drive system located at surface. The drive system generally includes a rotary table or a top drive, and the drill string includes a lower end part of increased weight, i.e. the bottom hole assembly (BHA) which provides the necessary weight on bit during drilling. By a top drive is meant a drive system which drives the drill string in rotation at its upper end, i.e. close to where the string is suspended from the drilling rig. In view of the length of the drill string, which is in many cases of the order of 3000 m or more, the drill string is subjected to considerable elastic deformations including twist around its longitudinal axis whereby the BHA is twisted relative to the upper end of the string. Each of the rotary table, the top drive and the BHA has a certain moment of inertia, therefore the elastic twist of the drill string leads to rotational vibrations resulting in considerable speed variations of the drill bit at the lower end of the string. One particularly unfavourable mode of drill string behaviour is stick-slip whereby the rotational speed of the drill bit cyclicly decreases to zero, followed by increasing torque of the string due to continuous rotation by the drive system and corresponding accumulation of elastic energy in the drill string, followed by coming loose of the drill string and acceleration up to speeds significantly higher than the nominal rotational speed of the drive system.
The large speed variations induce large torque variations in the drill string, leading to adverse effects such as damage to the string tubulars and the bit, and a reduced rate of penetration into the rock formation.
To suppress the stick-slip phenomenon, control systems have been applied to control the speed of the drive system such that the rotational speed variations of the drill bit are damped. One such system is disclosed in EP-B-443 689, in which the energy flow through the drive system of the drilling assembly is controlled to be between selected limits, the energy flow being definable as the product of an across-variable and a through-variable. The speed fluctuations are reduced by measuring at least one of the variables and adjusting the other variable in response to the measurement.
It is an object of the invention to provide a system for drilling a borehole in an earth formation, which system has a reduced tendency of stick-slip of the drill string in the borehole.